diff --git a/package-lock.json b/package-lock.json
index 0f0519f1..c4426415 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -9,7 +9,7 @@
       "version": "0.88.0",
       "license": "Apache-2.0",
       "dependencies": {
-        "@agentclientprotocol/sdk": "1.7.0",
+        "@agentclientprotocol/sdk": "1.8.0",
         "@anthropic-ai/claude-agent-sdk": "0.3.293",
         "diff": "9.0.0",
         "zod": "4.6.5"
@@ -39,9 +39,9 @@
       }
     },
     "node_modules/@agentclientprotocol/sdk": {
-      "version": "1.7.0",
-      "resolved": "https://registry.npmjs.org/@agentclientprotocol/sdk/-/sdk-1.7.0.tgz",
-      "integrity": "sha512-ZORuxsuEnLjRly7t6Ad7zAsWitjWsyprswyPUHz/yycPb+tTm3DGa8XheaHhRXqX8LS9snuxaYavXSRA50TlHw==",
+      "version": "1.8.0",
+      "resolved": "https://registry.npmjs.org/@agentclientprotocol/sdk/-/sdk-1.8.0.tgz",
+      "integrity": "sha512-fbP7qHZsFyTJVT0/RPrcWJ8An26qLXHEuGCHHEKDyxD1xpXV+Ejyh2LAE6vBAFePkpVjZeeohnbgvuEhZ7wLMg==",
       "license": "Apache-2.0",
       "peerDependencies": {
         "zod": "^3.25.0 || ^4.0.0"
diff --git a/package.json b/package.json
index 7a0da34a..d68b28c6 100644
--- a/package.json
+++ b/package.json
@@ -65,7 +65,7 @@
     "node": ">=22"
   },
   "dependencies": {
-    "@agentclientprotocol/sdk": "1.7.0",
+    "@agentclientprotocol/sdk": "1.8.0",
     "@anthropic-ai/claude-agent-sdk": "0.3.293",
     "diff": "9.0.0",
     "zod": "4.6.5"
diff --git a/src/v2/prompt.ts b/src/v2/prompt.ts
index fc1b5abd..df15e195 100644
--- a/src/v2/prompt.ts
+++ b/src/v2/prompt.ts
@@ -14,7 +14,7 @@ import {
 import * as v2 from "@agentclientprotocol/sdk/experimental/v2";
 import type { ClaudeAcpAgent } from "../acp-agent.js";
 import { splitNoticeText } from "../session-notices.js";
-import type { TurnEvents, TurnOutcome } from "../turn-events.js";
+import type { StopReason, TurnEvents, TurnOutcome } from "../turn-events.js";
 
 /**
  * Starts a turn for `params`, and answers when Claude Code takes the prompt
@@ -126,14 +126,19 @@ function jsonRpcError(error: unknown): { code: number; message: string; data?: u
   };
 }
 
+/**
+ * The `idle` of a turn that ended. Every stop reason the agent reports is one
+ * of v2's own, so the state is typed as those members of `IdleStateUpdate`:
+ * the type checker matches each stop reason to its member, which it cannot do
+ * against the whole union with its custom and unknown members.
+ */
 function idle({ stopReason, usage, _meta }: TurnOutcome): v2.SessionUpdate {
-  return {
-    sessionUpdate: "state_update",
-    state: "idle",
+  const state: Extract<v2.IdleStateUpdate, { stopReason: StopReason }> = {
     stopReason,
     ...(usage ? { usage } : {}),
     ...(_meta ? { _meta } : {}),
   };
+  return { sessionUpdate: "state_update", state: "idle", ...state };
 }
 
 /**